1. Field of the Invention
The present invention relates generally to a medical imaging apparatus and more particularly to an X-ray computer tomograph (hereinafter referred to as "X-ray CT apparatus") for imaging a subject in synchronism with the respiration of the subject.
2. Description of the Related Art
There is a well-known conventional method of acquiring a tomographic image of a subject placed on a bed by means of an X-ray CT apparatus or one of medical imaging apparatus. With the X-ray CT apparatus, an operator operates a control table to slide a movable top plate provided above the bed and to introduce the subject into an opening formed in a central portion of a CT apparatus frame having an imaging device comprising an X-ray tube. After the top plate is situated at a predetermined position, a scanning operation of a predetermined scanning system is performed. An image is reconstructed on the basis of acquired image data, and an image is displayed on a monitor.
The scanning system is classified into two systems: 1) a single scan system in which image data of one tomogram is acquired with the top plate kept stationary, and 2) a multi-scan system in which image data of many tomograms is obtained by a single operation. Further, the multi-scan system is classified into two systems: 1) a helical scan system in which image data is acquired helically along the axis of a subject, and 2) a rapid scan system in which image data is acquired by repeating at short time intervals the above-mentioned single scan system and the movement of the top plate. A three-dimensional (3D) image is formed based on many tomograms obtained by the multi-scan system.
In the X-ray CT apparatus, an artifact may appear in an image due to movement of the subject during scanning. In any of the scan systems, therefore, the subject must hold this breath from the beginning to the end of the scan. In the case where locations to be imaged are present in a wide area, if the subject takes a breath during the scan operation, the scan operation must be suspended. Then, the subject holds his breath once again and the scanning operation is resumed from the location at which the scan operation was suspended due to the breathing of the subject.
The above conventional X-ray CT apparatus has the following problem. The positional relationship between the subject's muscles and internal organs varies according to the state of respiration (hereinafter referred to as "respiration depth") of the subject. Thus, the subject must make constant the respiration depth before and after the breathing spell. However, since the respiration depth is determined by the subject's sensation, it is very difficult for the subject to make exactly constant the respiration depth before and after the breathing spell.
If the subject takes a breath during acquiring image data in the helical scan system, image data becomes discontinuous before and after the breathing spell, and it is not possible to form a tomogram of an area including a location at which the scan operation was suspended due to the breathing spell. Consequently, the scan operation must be performed once again from the beginning, and the amount of radiation applied to the subject increases.
In the case where a plurality of tomograms are formed on the basis of projection image data obtained by the multi-scan system (e.g. rapid scan system), if the positional relationship between the internal organs varies before and after the respiration spell, an object such as an ulcer, which is smaller than the internal organs, may move in a direction perpendicular to a slice plane in some cases. Thus, it may be possible that the object does not appear on a tomogram or the size of the object is not exactly determined.
When a 3D image is constructed from many tomograms obtained by the multi-scan system, discontinuity may appear on a 3D image between locations imaged before and after the breathing spell and the obtained 3D image becomes unnatural.
On the other hand, in the case where tomograms of a specified location of the subject are taken at regular time intervals in order for a doctor to diagnose the healing condition of the affected part before and after a surgical operation, the positional relationship between the internal organs, etc. varies if the respiration depth varies. As a result, exact observation and diagnosis cannot be performed.
In addition to the above X-ray CT apparatus, other medical imaging apparatus have the same problems.